1. Field of the Invention
The present invention relates to a plasma processing apparatus for applying plasma processing to an article to be processed (hereinafter, simply referred to as “article” as occasion demands) using microwaves, and more particularly to a microwave applicator having a circular (or annular) waveguide, a plasma processing apparatus provided therewith, and a plasma processing method.
2. Related Background Art
As plasma processing apparatuses that use microwaves as an excitation source for plasma excitation, there have been known the plasma polymerizing apparatus, the CVD apparatus, the surface modifying apparatus, the etching apparatus, the ashing apparatus, and the cleaning apparatus and the like.
The CVD using such a so-called microwave plasma processing apparatus is carried out, for example, as follows. A gas is introduced into a plasma generation chamber and/or a film formation chamber of a microwave plasma CVD apparatus, and a microwave energy is simultaneously applied to generate a plasma in the plasma generation chamber to form ions or radicals through excitation, decomposition, ionization, or the like of the gas, thereby forming a deposited film on an article disposed in the plasma generation chamber or the film formation chamber apart from the plasma generation chamber. Further, a similar method can be used to carry out plasma polymerization or surface modification such as oxidation, nitridation or fluorination of an organic substance.
Furthermore, the etching of an article using a so-called microwave plasma etching apparatus is carried out, for example, as follows. An etchant gas is introduced into a processing chamber of the apparatus, and a microwave energy is simultaneously applied to generate a plasma in the processing chamber to thereby form ions, radicals or the like through excitation, decomposition, or ionization of the etchant gas, thereby etching a surface of an article disposed in the processing chamber with the thus formed ions, radicals or the like.
In addition, the ashing of an article using a so-called microwave plasma ashing apparatus is carried out, for example, as follows. An ashing gas is introduced into a processing chamber in the apparatus, and a microwave energy is simultaneously applied to generate a plasma in the processing chamber to thereby form ions, radicals, ozone or the like through excitation, decomposition, or ionization of the ashing gas, thereby ashing a surface of an article, namely a photoresist disposed in the processing chamber. As with the ashing, it is possible to effect cleaning for removing unwanted matter deposited on a to-be-processed surface of an article.
In the microwave plasma processing apparatus, since microwaves are used as a gas excitation source, electrons can be accelerated by an electric field having a high frequency, thereby efficiently ionizing or exciting gas molecules. Thus, the microwave plasma processing apparatus is advantageous in that the efficiency of ionization, excitation, and decomposition of a gas is high, so that a high density plasma can relatively easily be formed, and that it is possible to carry out fast, high quality processing at a low temperature. In addition, there is a further advantage that the microwaves have a property of penetrating a dielectric member such as quartz glass, so that the plasma processing apparatus can be constituted as a electrodeless discharge type, whereby highly clean plasma processing can be carried out.
To increase the processing speed of such a microwave plasma processing apparatus, plasma processing apparatuses utilizing electron cyclotron resonance (ECR) have been put to practical use. The ECR is a phenomenon in which when the magnetic flux density is 87.5 mT, the electron cyclotron frequency for electrons rotating around the magnetic line of force is brought into conformity with the general frequency of the microwaves of 2.45 GHz, whereby the electrons resonantly absorb microwaves to be accelerated, thereby generating a high density plasma.
Further, there have been proposed other types of plasma processing apparatuses for generating a high density plasma.
For example, U.S. Pat. No. 5,034,086 discloses a plasma processing apparatus using a radial line slot antenna (RLSA).
In addition, Japanese Patent Application Laid-Open No. 5-290995, U.S. Pat. No. 5,359,177, and EP 0564359 disclose plasma processing apparatuses using a circular waveguide with terminals.
Separately, as an example of a microwave plasma processing apparatus, there has recently been proposed an apparatus using an endless circular waveguide in which a plurality of slots are formed on an inner side surface thereof as a device for uniform and efficient introduction of microwaves (Japanese Patent Application Laid-Open No. 5-345982; U.S. Pat. No. 5,538,699).
However, when the conventional microwave plasma processing apparatus provided with an endless circular waveguide having slots on an inner side surface thereof is used to effect processing in a high pressure region at 100 mTorr (about 13.3 Pa) or more, as in the case of the ashing processing, the diffusion of plasma is suppressed, so that the plasma may locally exist in the periphery of the chamber to reduce the processing speed for the center portion of the article. In addition, the volume of the plasma generation space is required to be very large.
Further, Japanese Patent Application Laid-Open No. 7-90591 discloses a plasma processing apparatus using a disc-like microwave introducing device. In this apparatus, a gas is introduced into a waveguide and emitted to a plasma generation chamber through slots provided in the waveguide.
Compared with these conventional apparatuses, the plasma processing apparatus previously proposed by the present inventors has a configuration as shown in FIG. 12.
In FIG. 12, reference numeral 1 designates a container (or vessel) which can be evacuated; 2 is a holding means for holding an article to be processed; 3 is a microwave supply means (also referred to as “microwave applicator”) comprising a circular hollow waveguide having a circular waveguide therein; 4 is a dielectric window; and 7 is a gas supply pipe having gas supply ports 7a. In the apparatus configured using these components, microwaves are introduced into the microwave applicator 3 through a microwave introducing port 15 and supplied from slots 3b through the dielectric window 4 into the container 1.
FIGS. 13, 14 and 15 are schematic views illustrating the propagation of microwaves through the circular waveguide of the microwave applicator and the radiation of microwaves through the slots.
FIG. 13 shows the circular waveguide as seen from above with the slots omitted, FIG. 14 shows a cross section taken along line 14—14, and FIG. 15 shows a cross section taken along line 15—15.
The vicinity of the microwave introducing port 15 forms an equivalent circuit of E-plane T-junction (or T-distribution), and microwaves introduced through the microwave introducing port 15 have their course changed so as to fork clockwise d2 and counterclockwise d1. Each slot 3b is provided so as to intersect the microwave traveling directions d1 and d2 so that the microwaves travel while being emitted through the slots.
Since the circular waveguide has no terminals and is endless, the microwaves propagating in the directions d1 and d2 (z-axis direction) interfere mutually. Reference numeral C1 denotes an annulus (ring) formed by connecting width-wise centers of the waveguide, and the standing waves of a predetermined mode can be generated more easily by setting the length of this ring, that is, the circumferential length at an integral multiple of the guide wavelength (wavelength in waveguide).
FIG. 14 shows a cross section perpendicular to the microwave traveling direction (z-axis direction). In this figure, the upper and bottom surfaces 3c of the waveguide form H-planes perpendicular to the direction of electric field EF, while the right and left surfaces 3d of the waveguide form E-planes parallel to the direction of the electric field EF. Reference numeral C0 denotes the center of the longitudinal direction of the slot 3b, that is, the direction (x-axis direction) perpendicular to the microwave traveling/propagating direction.
Thus, the cross section of the waveguide that is perpendicular to the microwave traveling direction has a rectangular shape having the x- and the y-axes as the longer and the shorter sides, respectively.
Microwaves MW introduced into the circular waveguide 3a are distributed by the distributor 10 of E-plane T-junction to the right and left of the drawing and propagate at a guide wavelength longer than the wavelength in the free space. The distributed microwaves interfere with each other at their opposing portion to form standing waves at every ½ of the guide wavelength. Leakage waves EW radiated through the dielectric window 4 from the slots 3b provided at such positions as to maximize the electric field crossing the slots generate a plasma P1 near the slots 3b. When the electron frequency of the generated plasma P1 exceeds the frequency of the microwave power source (for example, when the electron density exceeds 7×1010 cm−3 at the power source frequency of 2.45 GHz), the so-called cut-off in which microwaves can not propagate through the plasma is caused, so that they propagate through the interface between the dielectric window 4 and the plasma as surface waves SW. Surface waves SW introduced via adjacent slots interfere with each other to form antinodes (loops) of electric field at every ½ of the wavelength (λ∈r−1/2 wherein λ is microwave wavelength in free space, and ∈r is dielectric constant) of the surface waves SW. The antinodes of electric field resulting from the interference of the surface waves leaked to the plasma generation space side generate a surface-wave interfered plasma (SIP) P2. At this time, when a processing gas is introduced into the plasma processing chamber, the processing gas is excited, decomposed, or ionized by the thus generated high density plasma to enable processing of a surface of an article.
The use of such a microwave plasma processing apparatus can generate a high density, low potential plasma of a uniformity within ±3%, an electron density 1012/cm3 or more, an electron temperature 3 eV or less, and a plasma potential 20 V or less in a space with an aperture of a diameter of 300 mm or more at a pressure of about 1.33 Pa and a microwave power of 1 kW or more.
Thus, the gas can fully be reacted and supplied in an active state to a surface to be processed. Furthermore, when the pressure is 2.7 Pa and the microwave power is 2 kW, any current due to the microwaves cannot be detected at a location apart by 8-10 mm away from the inner surface of the dielectric window. This means that a very thin plasma layer is formed near the dielectric window in a high pressure region where plasma diffusion is suppressed. Thus, article surface damage due to incident ions can be reduced, thereby enabling high quality and high speed processing even at low temperatures.
Incidentally, the circumferential length of the circular waveguide must be selected from 2 times, 3 times, 4 times, . . . the guide wavelength (i.e., integral multiple of the guide wavelength) depending the processing area of an article. When air exists at atmospheric pressure in the waveguide, considering that the guide wavelength is about 159 mm, selectable circumferential length is about 318 mm, about 477 mm, about 636 mm, . . . Converting these values to the diameter of the ring provides about 101 mm, about 151 mm, about 202 mm.
On the other hand, when an ordinary 8-inch or 12-inch wafer is used as an article to be processed, the diameters thereof are about 200 mm and about 300 mm, respectively. Even when an optimum combination of the both members, it can not be said that uniformity of plasma and uniformity of processing are attained sufficiently. For example, there is caused a phenomenon in which the plasma density is lowered near the center of the ring or near the center of the article, so that the processing speed is decreased.